The overall objectives of this research are to develop and validate systems for direct in vivo mutagenicity testing in man. The primary system to be improved and validated is our thioguanine-resistant peripheral blood lymphocyte variant frequency (TG(r) PBL Vf) assay. This assay enumerates PHA stimulated PBL's able to incorporate 3H-thymidine in the presence of the cytotoxic purine base analog thioguanine. The mutant nature of the TG(r) PBL's in non-Lesch-Nyhan individuals will be investigated using somatic cell hybridization techniques. The assay will be used to measure possible increases in mutant cell frequencies in individuals exposed to mutagens or carcinogens. Clinical follow-up will be performed to look for correlation between elevated TG(r) PBL Vf values and malignancies or birth defects. Two other groups of individuals, patients undergoing immunological reconstitution with cultured thymic epithelium transplantation and individuals with a family history of excessive cancer incidence, will also be studied. Further effort will be directed towards using other cell types, other genetic markers, and adapting the TG(r) PBL assay to an animal model.